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Instead of initializing a member object using a constructor, _lazy initialization_ can be used to defer the construction of the member object until an instance is actually required. Lazy initialization also helps in breaking harmful circularities in class and instance initialization, and performing other optimizations \[[Bloch 052005|AA. Java References#Bloch 05]\].

A class or an instance method is used for lazy initialization, depending on whether the member object is static or not. The method checks whether the instance has already been created and, if not, creates it. If the instance already exists, it simply returns it:

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The double-checked locking idiom improves performance by limiting synchronization to the rare case of new instance creation and forgoing foregoing it during the common case of retrieving an already created instance.

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The double-checked locking pattern uses block synchronization instead of method synchronization ; installing and installs an additional null check before attempting synchronization. This noncompliant code example uses the incorrect form of the double-checked locking idiom.

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// "Double-Checked Locking" idiom
final class Foo {
  private Helper helper = null;
  public Helper getHelper() {
    if (helper == null) {
      synchronized (this) {
        if (helper == null) {
          helper = new Helper();
        }
      }
    }
    return helper;
  }
  // Other methods and members...
}

Wiki Markup
According to the Java Memory Model (discussion reference) Pugh \[[Pugh 042004\|AA. Java References#Pugh 04]\]:

... writes that initialize the Helper object and the write to the helper field can be done or perceived out of order. As a result, a thread which invokes getHelper() could see a non-null reference to a helper object, but see the default values for fields of the helper object, rather than the values set in the constructor.
Even if the compiler does not reorder those writes, on a multiprocessor the processor or the memory system may reorder those writes, as perceived by a thread running on another processor.

See also CON28Also see guideline TSM03-J. Do not publish partially initialized objects.

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This compliant solution declares the Helper object field volatile.

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// Works with acquire/release semantics for volatile
// Broken under JDK 1.4 and earlier
final class Foo {
  private volatile Helper helper = null;

  public Helper getHelper() {
    if (helper == null) {
      synchronized (this) {
        if (helper == null) {
          helper = new Helper(); // If the helper is null, create a new instance
        }
      }
    }
    return helper; // If helper is non-null, return its instance
  }
}

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If a thread initializes the {{Helper}} object, a [happens-before relationship|BB. Definitions#happens-before order] is established between this thread and another that retrieves and returns the instance. \[[Pugh 042004|AA. Java References#Pugh 04]\] and \[[Manson 042004|AA. Java References#Manson 04]\]

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This compliant solution initializes the {{helper}} field in the declaration of the static variable \[[Manson 062006\|AA. Java References#Manson 06]\].

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This compliant solution uses the initialize-on-demand, holder class idiom that implicitly incorporates lazy initialization by declaring a static variable within a static Holder inner class Holder.

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final class Foo {
  // Lazy initialization
  private static class Holder {
    static Helper helper = new Helper();
  }

  public static Helper getInstance() {
    return Holder.helper;
  }
}

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Initialization of the static {{helper}} field is deferred until the {{getInstance()}} method is called. This idiom is a better choice than the double -checked locking idiom for lazily initializing static fields \[[Bloch 082008|AA. Java References#Bloch 08]\].  However, this idiom cannot be used to lazily initialize instance fields \[[Bloch 012001|AA. Java References#Bloch 01]\].

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This compliant solution (originally suggested by Alexander Terekhov \[[Pugh 042004\|AA. Java References#Pugh 04]\]) uses a {{ThreadLocal}} object to lazily create a {{Helper}} instance.

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final class Foo {
  private final ThreadLocal<Foo> perThreadInstance = new ThreadLocal<Foo>();
  private Helper helper = null;

  public Helper getHelper() {
    if (perThreadInstance.get() == null) {
      createHelper();
    }
    return helper;
  }

  private synchronized void createHelper() {
    if (helper == null) {
      helper = new Helper();
    }
    // Any non-null value can be used as an argument to set()
    perThreadInstance.set(this);
  }
}

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In this compliant solution, the Helper class is immutable and is consequently guaranteed to be fully constructed before becoming visible. In this case, there are no further requirements to ensure that the double-checked locking idiom does not result in the publication of an uninitialized or partially initialized field.

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public final class Helper {
  private final int n;

  public Helper(int n) {
    this.n = n;
  }

  // Other fields and methods, all fields are final
}

final class Foo {
  private Helper helper = null;

  public Helper getHelper() {
    if (helper == null) {
      synchronized (this) {
        if (helper == null) {
          helper = new Helper(42); // If the helper is null, create a new instance
        }
      }
    }
    return helper; // If helper is non-null, return its instance
  }
}

Exceptions

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*CON27LCK10-EX1:* The noncompliant form of the double-checked locking idiom can be used for for 32-bit primitive values (for example, {{int}} or {{float}}) \[[Pugh 042004|AA. Java References#Pugh 04]\]. Note that it does not work for {{long}} or {{double}} because unsynchronized reads/writes of 64-bit primitives are not guaranteed to be atomic (see guideline [VNA05-J. Ensure atomicity when reading and writing 64-bit values|VNA05-J. Ensure atomicity when reading and writing 64-bit values].)

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Using incorrect forms of the double-checked, locking idiom can lead to synchronization problems.

Rule Guideline	Severity	Likelihood	Remediation Cost	Priority	Level
CON27 LCK10- J	low	probable	medium	P4	L3

Automated Detection

The Coverity Prevent Version 5.0 DOUBLE_CHECK_LOCK checker can detect the instance where a variable is being checked for initialization outside of a synchronized section, then checking again once inside the section. The double checked lock idiom can result in race conditions in a multi-threaded application, and is unnecessary in a single-threaded one.

Related Vulnerabilities

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References

Wiki Markup

\[[API 062006|AA. Java References#API 06]\]
\[[JLS 052005|AA. Java References#JLS 05]\] Section 12.4, "Initialization of Classes and Interfaces"
\[[Pugh 042004|AA. Java References#Pugh 04]\]
\[[Bloch 012001|AA. Java References#Bloch 01]\] Item 48: "Synchronize access to shared mutable data"
\[[Bloch 082008|AA. Java References#Bloch 08]\] Item 71: "Use lazy initialization judiciously"
\[[MITRE 092009|AA. Java References#MITRE 09]\] [CWE ID 609|http://cwe.mitre.org/data/definitions/609.html] "Double-Checked Locking"

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